Electrochemical cell having an electrode with a dicarbonate additive in the electrode active mixture

ABSTRACT

An electrochemical cell of either a primary or a secondary chemistry, is described. In either case, the cell has a negative electrode of lithium or of an anode material which is capable of intercalating and de-intercalating lithium coupled with a positive electrode of a cathode active material. A dicarbonate compound is mixed with either the anode material or the cathode active material prior to contact with its current collector. The resulting electrode couple is activated by a nonaqueous electrolyte. The electrolyte flows into and throughout the electrodes causing the dicarbonate additive to dissolve in the electrolyte. The dicarbonate solute is then able to contact the lithium to provide an electrically insulating and ionically conducting passivation layer thereon.

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002] The present invention generally relates to the conversion ofchemical energy to electrical energy, and more particularly, to anelectrochemical cell of either a primary or a secondary chemistry. Ineither case, the cell has a negative electrode of lithium or of an anodematerial which is capable of intercalating and de-intercalating lithiumcoupled with a positive electrode of a cathode active material. Adicarbonate compound is mixed with either the anode material or thecathode active material prior to contact with its current collector. Theresulting electrode couple is activated by a nonaqueous electrolyte. Theelectrolyte flows into and throughout the electrodes, causing thedicarbonate compound to dissolve in the electrolyte. The dicarbonatesolute is then able to contact the lithium to provide an electricallyinsulating and ionically conducting passivation layer thereon.

[0003] 2. Prior Art

[0004] In a primary cell, the formation of a surface film on an alkalimetal anode, especially when the anode is of lithium, is unavoidable.Therefore, the prior art in U.S. Pat. No. 6,063,526 to Gan et al.teaches providing a dicarbonate additive in the electrolyte tobeneficially modify the anode surface film of an alkali metal primarycell, particularly a lithium cell. The dicarbonate additive interactswith the lithium anode to form an ionically conductive surface layer ofa dicarbonate salt thereon. This salt is more conductive than lithiumoxide which may form on the anode in the absence of the dicarbonateadditive. In fact, it is believed that the lithium dicarbonate or thelithium salt of the dicarbonate reduction product on the surface of theanode provides for the existence of charge delocalization due toresonance equilibration at the anode surface. This equilibration allowslithium ions to travel easily from one molecule to the other via alithium ion exchange mechanism. As a result, beneficial ionicconductance is realized. Similarly, U.S. Pat. No. 6,174,629 to Gan etal. describes the provision of a dicarbonate additive in the electrolyteof a secondary cell.

[0005] However, the present invention is the first known attempt tointroduce dicarbonate additives into the chemistry of the cell by havingthem leach from the cathode active mixture of the positive electrode fora primary or a secondary cell or from the anode material of a secondarycell. Benefits to this approach are that the dicarbonate compound in asolid form is easily mixed with the electrode material and, if desired,a conductive diluent and a binder, to form a homogeneous mixture whichis easily fabricated into an electrode. A cell is formed when the thuslyfabricated negative electrode and positive electrode are activated withan electrolyte. The electrolyte serves to wet the electrode material,causing the dicarbonate additive to dissolve therein. Then, theelectrolyte becomes a vehicle for transport of the dicarbonate compoundfrom the host electrode to form an ionically conductive surface layer onthe lithium in a similar manner as if the dicarbonate compound had beenadded directly to the electrolyte according to the prior art. However,in contrast to the prior art Gan et al. patents, the electrode materialmixed with the dicarbonate additive serves to meter its beneficialeffects as it gradually leaches from the host electrode.

SUMMARY OF THE INVENTION

[0006] The present invention relates to both primary and secondaryelectrochemical cells. An exemplary primary cell is a nonaqueouselectrolyte, alkali metal/mixed metal oxide electrochemical cell and, inparticular, a lithium/silver vanadium oxide electrochemical cell.Lithium/silver vanadium oxide cells are designed for current pulsedischarge applications required in powering an implantable medicaldevice such as a cardiac defibrillator. A defibrillator requires a cellthat may run under a light load, device monitoring mode for extendedperiods of time interrupted by high rate, current pulse discharge duringdevice activation.

[0007] Voltage delay is a phenomenon typically exhibited in alithium/silver vanadium oxide cell that has been depleted of about 40%to about 70% of its capacity and is subjected to current pulse dischargeapplications. The occurrence of voltage delay is detrimental because itmay result in delayed device activation and shortened device life. Rdcbuild-up is characterized by an increase in cell resistance inlithium/silver vanadium oxide cells that have been depleted of about 50%to about 100% of their capacity. Rdc build-up also results in a loweringof pulse minimum voltages during high rate discharge, which in turnlimits the life of the electrochemical cell.

[0008] The desirable decrease in both voltage delay and Rdc build-up isrealized in primary cells that contain silver vanadium oxide having adicarbonate compound mixed therewith. The dicarbonate compound is mixedwith the cathode active material prior to the positive electrode beingassembled into the cell. The thusly fabricated positive electrode iselectrochemically coupled with a negative electrode and activated with anonaqueous electrolyte. The electrolyte permeates the positive electrodeto wet the cathode active material and serve as a vehicle for dissolvingand transporting the dicarbonate compound to the anode active material.In a primary cell, the dicarbonate compound reacts with the lithiumanode to form an ionically conductive protective film thereon.

[0009] In a secondary cell built in a discharged condition, thedicarbonate compound is mixed with either the cathode active material,preferably of lithium cobalt oxide, or the carbonaceous anode material.The dicarbonate compound reacts with the lithiated material of thepositive electrode and also when the lithium intercalates with the anodematerial of the negative electrode. The thusly formed dicarbonate saltat the solid electrolyte interface is responsible for improved cyclingefficiency in secondary cells.

[0010] These and other objects of the present invention will becomeincreasingly more apparent to those skilled in the art by reference tothe following description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] As used herein, the term “pulse” means a short burst ofelectrical current of a significantly greater amplitude than that of aprepulse current immediately prior to the pulse. A pulse train consistsof at least two pulses of electrical current delivered in relativelyshort succession with or without open circuit rest between the pulses. Atypical current pulse is of about 15.0 mA/cm² to about 35.0 mA/cm².

[0012] The electrochemical cell of the present invention is of either aprimary chemistry or a secondary, rechargeable chemistry. For both theprimary and secondary types, the cell comprises an anode active metalselected from Groups IA, IIA and IIIB of the Periodic Table of theElements, including lithium, sodium, potassium, etc., and their alloysand intermetallic compounds including, for example, Li—Si, Li—Al, Li—Band Li—Si—B alloys and intermetallic compounds. The preferred metalcomprises lithium. An alternate negative electrode comprises a lithiumalloy, such as lithium-aluminum alloy. The greater the amount ofaluminum present by weight in the alloy, however, the lower the energydensity of the cell.

[0013] For a primary cell, the anode is a thin metal sheet or foil ofthe lithium material, pressed or rolled on a metallic anode currentcollector, i.e., preferably comprising nickel, to form the negativeelectrode. In the exemplary cell of the present invention, the negativeelectrode has an extended tab or lead of the same material as thecurrent collector, i.e., preferably nickel, integrally formed therewithsuch as by welding and contacted by a weld to a cell case of conductivematerial in a case-negative electrical configuration. Alternatively, thenegative electrode may be formed in some other geometry, such as abobbin shape, cylinder or pellet to allow an alternate low surface celldesign.

[0014] In secondary electrochemical systems, the anode or negativeelectrode comprises an anode material capable of intercalating andde-intercalating the anode active material, such as the preferred alkalimetal lithium. A carbonaceous negative electrode comprising any of thevarious forms of carbon (e.g., coke, graphite, acetylene black, carbonblack, glassy carbon, etc.) which are capable of reversibly retainingthe lithium species is preferred for the anode material. A “hairycarbon” material is particularly preferred due to its relatively highlithium-retention capacity. “Hairy carbon” is a material described inU.S. Pat. No. 5,443,928 to Takeuchi et al., which is assigned to theassignee of the present invention and incorporated herein by reference.Graphite is another preferred material. Regardless of the form of thecarbon, fibers of the carbonaceous material are particularlyadvantageous because they have excellent mechanical properties whichpermit them to be fabricated into rigid electrodes that are capable ofwithstanding degradation during repeated charge/discharge cycling.Moreover, the high surface area of carbon fibers allows for rapidcharge/discharge rates.

[0015] A typical negative electrode for a secondary cell is fabricatedby mixing about 90 to 97 weight percent “hairy carbon” or graphite withabout 3 to 10 weight percent of a binder material, which is preferably afluoro-resin powder such as polytetrafluoroethylene (PTFE),polyvinylidene fluoride (PVDF), polyethylenetetrafluoroethylene (ETFE),polyamides, polyimides, and mixtures thereof. This negative electrodeadmixture is provided on a current collector such as of a nickel,stainless steel, or copper foil or screen by casting, pressing, rollingor otherwise contacting the admixture thereto.

[0016] In either the primary cell or the secondary cell, the reaction atthe positive electrode involves conversion of ions which migrate fromthe negative electrode to the positive electrode into atomic ormolecular forms. For a primary cell, the cathode active materialcomprises at least a first transition metal chalcogenide constituentwhich may be a metal, a metal oxide, or a mixed metal oxide comprisingat least a first and a second metals or their oxides and possibly athird metal or metal oxide, or a mixture of a first and a second metalsor their metal oxides incorporated in the matrix of a host metal oxide.The cathode active material may also comprise a metal sulfide.

[0017] The metal oxide or the mixed metal oxide can be produced by thechemical addition, reaction, or otherwise intimate contact of variousmetal oxides and/or metal elements, preferably during thermal treatmentor chemical vapor deposition in mixed states. The active materialsthereby produced contain metals, oxides and sulfides of Groups IB, IIB,IIIB, IVB, VB, VIB, VIIB, and VIII of the Periodic Table of Elements,which includes the noble metals and/or other oxide compounds.

[0018] By way of illustration, and in no way intended to be limiting, anexemplary cathode active material comprises silver vanadium oxide havingthe general formula Ag_(x)V₂O_(y) in any one of its many phases, i.e.β-phase silver vanadium oxide having in the general formula x=0.35 andy=5.18, γ-phase silver vanadium oxide having in the general formulax=0.80 and y=5.4 and ε-phase silver vanadium oxide having in the generalformula x=1.0 and y=5.5, and combination and mixtures of phases thereof.For a more detailed description of silver vanadium oxide materials,reference is made to U.S. Pat. No. 4,310,609 to Liang et al., U.S. Pat.No. 5,389,472 to Takeuchi et al., U.S. Pat. No. 5,498,494 to Takeuchi etal. and U.S. Pat. No. 5,695,892 to Leising et al., all of which areassigned to the assignee of the present invention and incorporatedherein by reference.

[0019] Another preferred transition metal oxide useful with the presentinvention is a composite cathode active material that includes V₂O_(Z)wherein z≦5 combined with Ag₂O with the silver in either the silver(II),silver(I) or silver(0) oxidation state and CuO with the copper in eitherthe copper(II), copper(I) or copper(0) oxidation state to provide themixed metal oxide having the general formula Cu_(x)Ag_(y)V₂O_(z),(CSVO). Thus, this composite cathode active material may be described asa metal oxide-metal oxide-metal oxide, a metal-metal oxide-metal oxide,or a metal-metal-metal oxide and the range of material compositionsfound for Cu_(x)Ag_(y)V₂O_(z) is preferably about 0.01≦x≦1.0, about0.01≦y≦1.0 and about 5.01≦z≦6.5. Typical forms of CSVO areCu_(0.16)Ag_(0.67)V₂O_(z) with z being about 5.5 andCu_(0.5)Ag_(0.5)V₂O_(z) with z being about 5.75. The oxygen content isdesignated by z since the exact stoichiometric proportion of oxygen inCSVO can vary depending on whether the cathode active material isprepared in an oxidizing atmosphere such as air or oxygen, or in aninert atmosphere such as argon, nitrogen and helium. For a more detaileddescription of this cathode active material, reference is made to U.S.Pat. No. 5,472,810 to Takeuchi et al. and U.S. Pat. No. 5,516,340 toTakeuchi et al., both of which are assigned to the assignee of thepresent invention and incorporated herein by reference.

[0020] Additional cathode active materials for a primary cell includemanganese dioxide, cobalt oxide, nickel oxide, copper vanadium oxide,titanium disulfide, copper oxide, copper sulfide, iron sulfide, irondisulfide, and mixtures thereof.

[0021] In secondary cells, the positive electrode preferably comprises alithiated material that is stable in air and readily handled. Examplesof such air-stable lithiated cathode active materials include oxides,sulfides, selenides, and tellurides of such metals as vanadium,titanium, chromium, copper, molybdenum, niobium, iron, nickel, cobaltand manganese. The more preferred oxides include LiNiO₂, LiMn₂O₄,LiCoO₂, LiCo_(0.92)Sn_(0.08)O₂ and LiCo_(1-x)Ni_(x)O₂.

[0022] To discharge such secondary cells, the lithium metal comprisingthe positive electrode is intercalated into the carbonaceous negativeelectrode by applying an externally generated electrical potential torecharge the cell. The applied recharging electrical potential serves todraw lithium ions from the cathode active material, through theelectrolyte and into the carbonaceous material of the negative electrodeto saturate the carbon. The resulting Li_(x)C₆ negative electrode canhave an x ranging from about 0.1 to about 1.0. The cell is then providedwith an electrical potential and is discharged in a normal manner.

[0023] An alternate secondary cell construction comprises intercalatingthe carbonaceous material with the active lithium material before thenegative electrode is incorporated into the cell. In this case, thepositive electrode body can be solid and comprise, but not be limitedto, such active materials as manganese dioxide, silver vanadium oxide,titanium disulfide, copper oxide, copper sulfide, iron sulfide, irondisulfide and fluorinated carbon. However, this approach is compromisedby problems associated with handling lithiated carbon outside the cell.Lithiated carbon tends to react when contacted by air or water.

[0024] The above described cathode active materials, whether of aprimary or a secondary chemistry, are formed into an electrode body forincorporation into an electrochemical cell by mixing one or more of themwith a conductive additive such as acetylene black, carbon black and/orgraphite. Metallic materials such as nickel, aluminum, titanium andstainless steel in powder form are also useful as conductive diluentswhen mixed with the above listed active materials. The positiveelectrode of both a primary and a secondary cell further comprises abinder material which is preferably a fluoro-resin powder such aspowdered polytetrafluoroethylene (PTFE) or powdered polyvinylidenefluoride (PVDF). More specifically, a preferred cathode active materialfor a primary cell comprises SVO in any one of its many phases, ormixtures thereof, and/or CSVO mixed with a binder material and aconductive diluent. A preferred cathode active material for a secondarycell comprises lithium cobalt oxide mixed with a binder material and aconductive diluent.

[0025] In primary cells, the addition of at least one of a group ofdicarbonate additives to the cathode active mixture has beneficialeffects when the positive electrode is coupled to a negative electrodeand activated by a nonaqueous electrolyte. This causes the dicarbonateadditive to dissolve as a solute in the electrolyte to consequentlyminimize or eliminate voltage delay and reduce Rdc build-up when thecell is subjected to current pulse discharge conditions. For secondarysystems, the dicarbonate additive is provided in either the cathodeactive mixture or mixed with the carbonaceous anode material to benefitcycling efficiency.

[0026] The dicarbonate additive preferably has the formula:(R¹O)C(═O)OC(═O)(OR²), wherein R¹ and R² are the same or different andthey can both be a hydrogen atom or one of R¹ and R² is a saturated orunsaturated organic group if the other of R¹ and R² is H or anunsaturated organic group and wherein when any one of R¹ and R² is anunsaturated organic group, the unsaturated organic group contains 2 to13 carbon atoms and has the structure (R³)(R⁴)(R⁵)C— with at least R³being an aromatic substituent or an unsaturated organic or inorganicgroup and R¹ and R⁵ being a hydrogen atom or a saturated or unsaturatedhydrocarbon or heteroatom group, wherein when R¹ and R² are the same,they cannot both be saturated organic groups. The greatest effect isfound when the dicarbonate additive is selected from the groupconsisting of dibenzyl dicarbonate, diallyl dicarbonate, methyl benzyldicarbonate, ethyl benzyl dicarbonate, propyl benzyl dicarbonate, butylbenzyl dicarbonate, methyl allyl dicarbonate, ethyl allyl dicarbonate,propyl allyl dicarbonate, mono-allyl dicarbonate, mono-methyldicarbonate, mono-ethyl dicarbonate, mono-butyl dicarbonate, mono-propyldicarbonate, mono-benzyl dicarbonate, cyanomethyl methyl dicarbonate,nitromethyl methyl dicarbonate, and mixtures thereof. Preferably, theadditive is present in a range of about 0.05% to about 5.0%, by weight.

[0027] The above listed dicarbonate compounds are only intended to beexemplary of those that are useful with the present invention, and arenot to be construed as limiting. Those skilled in the art will readilyrecognize compounds which come under the purview of the general formulasset forth above and which will be useful as additives to reduce voltagedelay and Rdc build-up according to the present invention.

[0028] In that respect, a preferred positive electrode active admixtureaccording to the present invention comprises from about 80% to 99%, byweight, of a cathode active material comprising either one or both ofthe SVO and CSVO materials for a primary cell or lithium cobalt oxidefor a secondary cell mixed with a suitable binder, a conductive diluentand at least one of the above dicarbonate compounds. The resultingblended active mixture may be formed into a free-standing sheet prior tobeing contacted with a current collector to form the subject electrode.The manner in which the electrode mixture is prepared into afree-standing sheet is thoroughly described in U.S. Pat. No. 5,435,874to Takeuchi et al., which is assigned to the assignee of the presentinvention and incorporated herein by reference. Further, electrodecomponents for incorporation into both primary and secondary cells mayalso be prepared by rolling, spreading or pressing the electrode mixtureof the present invention onto a suitable current collector. Electrodesprepared as described above may be in the form of one or more platesoperatively associated with at least one or more plates of a counterelectrode, or in the form of a strip wound with a corresponding strip ofthe counter electrode in a structure similar to a “jellyroll”.

[0029] In order to prevent internal short circuit conditions, thepositive electrode is separated from the negative electrode by asuitable separator material. The separator is of electrically insulativematerial, and the separator material also is chemically unreactive withthe negative and positive electrode materials and both chemicallyunreactive with and insoluble in the electrolyte. In addition, theseparator material has a degree of porosity sufficient to allow flowtherethrough of the electrolyte during the electrochemical reaction ofthe cell. Illustrative separator materials include fabrics woven fromfluoropolymeric fibers including polyvinylidine fluoride,polyethylenetetrafluoroethylene, and polyethylenechlorotrifluoroethyleneused either alone or laminated with a fluoropolymeric microporous film,nonwoven glass, polypropylene, polyethylene, glass fiber materials,ceramics, a polytetrafluoroethylene membrane commercially availableunder the designation ZITEX (Chemplast Inc.), a polypropylene membranecommercially available under the designation CELGARD (Celanese PlasticCompany, Inc.) and a membrane commercially available under thedesignation DEXIGLAS (C.H. Dexter, Div., Dexter Corp.). The separatormay also be composed of non-woven glass, glass fiber materials andceramic materials.

[0030] The form of the separator typically is a sheet which is placedbetween the negative and positive electrodes and in a manner preventingphysical contact therebetween. Such is the case when the negativeelectrode is folded in a serpentine-like structure with a plurality ofpositive electrode plates disposed between the folds and received in acell casing or when the electrode combination is rolled or otherwiseformed into a cylindrical “jellyroll” configuration.

[0031] The primary and secondary electrochemical cells of the presentinvention further include a nonaqueous, ionically conductiveelectrolyte. The electrolyte serves as a medium for migration of ionsbetween the negative and the positive electrodes during theelectrochemical reactions of the cell, and nonaqueous solvents suitablefor the present invention are chosen so as to exhibit those physicalproperties necessary for ionic transport (low viscosity, low surfacetension and wettability). Suitable nonaqueous solvents are comprised ofan inorganic salt dissolved in a nonaqueous solvent system. For both aprimary and a secondary cell, the electrolyte preferably comprises analkali metal salt dissolved in a mixture of aprotic organic solventscomprising a low viscosity solvent including organic esters, ethers,dialkyl carbonates, and mixtures thereof, and a high permittivitysolvent including cyclic carbonates, cyclic esters, cyclic amides, andmixtures thereof. Low viscosity solvents include tetrahydrofuran (THF),diisopropylether, methyl acetate (MA), diglyme, triglyme, tetraglyme,1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), 1-ethoxy,2-methoxyethane (EME), dimethyl carbonate (DMC), diethyl carbonate(DEC), dipropyl carbonate (DPC), ethylmethyl carbonate (EMC),methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), and mixturesthereof. High permittivity solvents include propylene carbonate (PC),ethylene carbonate (EC), butylene carbonate (BC), acetonitrile, dimethylsulfoxide, dimethyl formamide, dimethyl acetamide, γ-valerolactone,γ-butyrolactone (GBL), N-methyl-pyrrolidinone (NMP), and mixturesthereof.

[0032] The preferred electrolyte for both a primary and a secondary cellcomprises a lithium salts selected from the group of LiPEF₆, LiBF₄,LiAsF₆, LiSbF₆, LiClO₄, LiAlCl₄, LiGaCl₄, LiC (SO₂CF₃) 3, LiN (SO₂CF₃)2, LiSCN, LiO₃SCF₂CF₃, LiC₆F₅SO₃, LiO₂CCF₃, LiSO₃F, LiNO₃, LiB (C₆H₅) 4,LiCF₃SO₃, and mixtures thereof. Suitable salt concentrations typicallyrange between about 0.8 to 1.5 molar.

[0033] In the present invention, the preferred primary electrochemicalcell has a negative electrode of lithium metal and a positive electrodeof the transition mixed metal oxide AgV₂O_(5.5) (SVO). For this primarycouple, the preferred activating electrolyte is 1.0M to 1.4M LiAsF₆dissolved in an aprotic solvent mixture comprising at least one of theabove listed low viscosity solvents and at least one of the above listedhigh permittivity solvents. The preferred aprotic solvent mixturecomprises a 50/50 mixture, by volume, of propylene carbonate and1,2-dimethoxyethane.

[0034] A preferred electrolyte for a secondary cell of a carbon/LiCoO₂couple comprises a solvent mixture of EC:DMC:EMC:DEC. Most preferredvolume percent ranges for the various carbonate solvents include EC inthe range of about 20% to about 50%; DMC in the range of about 12% toabout 75%; EMC in the range of about 5% to about 45%; and DEC in therange of about 3% to about 45%. In a preferred form of the presentinvention, the electrolyte activating the cell is at equilibrium withrespect to the ratio of DMC:EMC:DEC. This is important to maintainconsistent and reliable cycling characteristics. It is known that due tothe presence of low-potential (anode) materials in a charged cell, anun-equilibrated mixture of DMC:DEC in the presence of lithiated graphite(LiC₆˜0.01 V vs Li/Li⁺) results in a substantial amount of EMC beingformed. When the concentrations of DMC, DEC and EMC change, the cyclingcharacteristics and temperature rating of the cell change. Suchunpredictability is unacceptable. This phenomenon is described in detailin U.S. patent application Ser. No. 09/669,936, filed Sep. 26, 2000,which is assigned to the assignee of the present invention andincorporated herein by reference. Electrolytes containing the quaternarycarbonate mixture of the present invention exhibit freezing points below−50° C., and lithium ion secondary cells activated with such mixtureshave very good cycling behavior at room temperature as well as very gooddischarge and charge/discharge cycling behavior at temperatures below−40° C.

[0035] The assembly of the primary and secondary cells described hereinis preferably in the form of a wound element configuration. That is, thefabricated negative electrode, positive electrode and separator arewound together in a “jellyroll” type configuration or “wound elementcell stack” such that the negative electrode is on the outside of theroll to make electrical contact with the cell case in a case-negativeconfiguration. Using suitable top and bottom insulators, the wound cellstack is inserted into a metallic case of a suitable size dimension. Themetallic case may comprise materials such as stainless steel, mildsteel, nickel-plated mild steel, titanium, tantalum or aluminum, but notlimited thereto, so long as the metallic material is compatible for usewith components of the cell.

[0036] The cell header comprises a metallic disc-shaped body with afirst hole to accommodate a glass-to-metal seal/terminal pin feedthroughand a second hole for electrolyte filling. The glass used is of acorrosion resistant type having up to about 50% by weight silicon suchas CABAL 12, TA 23, FUSITE 425 or FUSITE 435. The positive terminal pinfeedthrough preferably comprises titanium although molybdenum, aluminum,nickel alloy, or stainless steel can also be used. The cell header istypically of a material similar to that of the case. The positiveterminal pin supported in the glass-to-metal seal is, in turn, supportedby the header, which is welded to the case containing the electrodestack. The cell is thereafter filled with the electrolyte solutiondescribed hereinabove and hermetically sealed such as by close-welding astainless steel ball over the fill hole, but not limited thereto.

[0037] The above assembly describes a case-negative cell, which is thepreferred construction of either the exemplary primary or secondary cellof the present invention. As is well known to those skilled in the art,the exemplary primary and secondary electrochemical systems of thepresent invention can also be constructed in case-positiveconfigurations.

[0038] It is appreciated that various modifications to the presentinventive concepts described herein may be apparent to those of ordinaryskill in the art without departing from the spirit and scope of thepresent invention as defined by the herein appended claims.

What is claimed is:
 1. An electrochemical cell comprising a negativeelectrode, a positive electrode and a nonaqueous electrolyte, theimprovement in the cell comprising: the negative electrode comprising ananode material contacted to an anode current collector and the positiveelectrode comprising a cathode active material contacted to a cathodecurrent collector, wherein at least one of the anode material and thecathode active material is mixed with a dicarbonate additive prior tobeing contacted to the respective anode current collector and cathodecurrent collector.
 2. The electrochemical cell of claim 1 as either aprimary or a secondary electrochemical cell.
 3. The electrochemical cellof claim 1 wherein the dicarbonate additive has the formula:(R¹O)C(═O)OC(═O)(OR²), wherein R¹ and R² are the same or different andthey can both be a hydrogen atom or one of R¹ and R² is a saturated orunsaturated organic group if the other of R¹ and R² is H or anunsaturated organic group and wherein when any one of R¹ and R² is anunsaturated organic group, the unsaturated organic group contains 2 to13 carbon atoms and has the structure (R³)(R⁴)(R⁵)C— with at least R³being an aromatic substituent or an unsaturated organic or inorganicgroup and R⁴ and R⁵ being a hydrogen atom or a saturated or unsaturatedhydrocarbon or heteroatom group, wherein when R¹ and R² are the same,they cannot both be saturated organic groups.
 4. The electrochemicalcell of claim 1 wherein the dicarbonate additive is selected from thegroup consisting of dibenzyl dicarbonate, diallyl dicarbonate, methylbenzyl dicarbonate, ethyl benzyl dicarbonate, propyl benzyl dicarbonate,butyl benzyl dicarbonate, methyl allyl dicarbonate, ethyl allyldicarbonate, propyl allyl dicarbonate, mono-allyl dicarbonate,mono-methyl dicarbonate, mono-ethyl dicarbonate, mono-butyl dicarbonate,mono-propyl dicarbonate, mono-benzyl dicarbonate, cyanomethyl methyldicarbonate, nitromethyl methyl dicarbonate, and mixtures thereof. 5.The electrochemical cell of claim 1 wherein the dicarbonate additive ispresent in at least one of the negative electrode and the positiveelectrode in a range of about 0.05% to about 5.0%, by weight.
 6. Theelectrochemical cell of claim 1 wherein the electrochemical cell is aprimary cell and the negative electrode is comprised of lithium or alithium-aluminum alloy.
 7. The electrochemical cell of claim 1 as aprimary cell and the cathode active material is selected from the groupconsisting of silver vanadium oxide, copper silver vanadium oxide,manganese dioxide, cobalt oxide, nickel oxide, copper oxide, coppersulfide, iron sulfide, iron disulfide, titanium disulfide, coppervanadium oxide, and mixtures thereof.
 8. The electrochemical cell ofclaim 7 wherein the positive electrode comprises from about 80 to about99 weight percent of the cathode active material.
 9. The electrochemicalcell of claim 8 wherein the positive electrode further comprises abinder material and a conductive additive.
 10. The electrochemical cellof claim 9 wherein the binder material is a fluoro-resin powder.
 11. Theelectrochemical cell of claim 9 wherein the conductive additive isselected from the group consisting of carbon, graphite powder, acetyleneblack, titanium powder, aluminum powder, nickel powder, stainless steelpowder, and mixtures thereof.
 12. The electrochemical cell of claim 1 asa secondary cell and the cathode active material is selected from thegroup consisting of oxides, sulfides, selenides, and tellurides ofmetals selected from the group consisting of vanadium, titanium,chromium, copper, molybdenum, niobium, iron, nickel, cobalt andmanganese.
 13. The electrochemical cell of claim 1 as a secondary celland the anode material is selected from the group consisting of coke,graphite, acetylene black, carbon black, glassy carbon, hairy carbon,and mixtures thereof.
 14. The electrochemical cell of claim 1 whereinthe electrolyte is a nonaqueous electrolyte and there is dissolvedtherein a lithium salt.
 15. The electrochemical cell of claim 1 whereinthe activated negative electrode and positive electrode provide theelectrochemical cell dischargeable to deliver at least one current pulseof an electrical current of a greater amplitude than that of a prepulsecurrent immediately prior to the current pulse.
 16. The electrochemicalcell of claim 15 wherein the current pulse is of about 15.0 mA/cm² toabout 35.0 mA/cm².
 17. The electrochemical cell of claim 1 associatedwith an implantable medical device powered by the cell.
 18. Incombination with an implantable medical device requiring at least onecurrent pulse for a medical device operating function, anelectrochemical cell which is dischargeable to deliver the currentpulse, the cell which comprises: a) a negative electrode; b) a positiveelectrode comprising a cathode active material contacted to a currentcollector, wherein the cathode active material is mixed with adicarbonate additive prior to being contacted to the current collector;and c) a nonaqueous electrolyte activating the negative electrode andpositive electrode, wherein the activated negative electrode andpositive electrode provide the electrochemical cell dischargeable todeliver at least one current pulse for the medical device operatingfunction, and wherein the current pulse is of an electrical current of agreater amplitude than that of a prepulse current immediately prior tothe current pulse.
 19. The combination of claim 18 wherein thedicarbonate additive has the formula: (R¹O)C(═O)OC(═O)(OR²), wherein R¹and R² are the same or different and they can both be a hydrogen atom orone of R¹ and R² is a saturated or unsaturated organic group if theother of R¹ and R² is H or an unsaturated organic group and wherein whenany one of R¹ and R² is an unsaturated organic group, the unsaturatedorganic group contains 2 to 13 carbon atoms and has the structure(R³)(R⁴)(R⁵)C— with at least R³ being an aromatic substituent or anunsaturated organic or inorganic group and R⁴ and R⁵ being a hydrogenatom or a saturated or unsaturated hydrocarbon or heteroatom group,wherein when R¹ and R² are the same, they cannot both be saturatedorganic groups.
 20. The combination of claim 18 wherein the dicarbonateadditive is selected from the group consisting of dibenzyl dicarbonate,diallyl dicarbonate, methyl benzyl dicarbonate, ethyl benzyldicarbonate, propyl benzyl dicarbonate, butyl benzyl dicarbonate, methylallyl dicarbonate, ethyl allyl dicarbonate, propyl allyl dicarbonate,mono-allyl dicarbonate, mono-methyl dicarbonate, mono-ethyl dicarbonate,mono-butyl dicarbonate, mono-propyl dicarbonate, mono-benzyldicarbonate, cyanomethyl methyl dicarbonate, nitromethyl methyldicarbonate, and mixtures thereof.
 21. The combination of claim 18wherein the dicarbonate additive is present in the positive electrode ina range of about 0.05% to about 5.0%, by weight.
 22. An electrochemicalcell, which comprises: a) an anode comprising lithium; b) a cathodecomprising silver vanadium oxide contacted to a current collector,wherein the silver vanadium oxide is mixed with a dicarbonate additiveprior to being contacted to the current collector; and c) a liquid,nonaqueous electrolyte operatively associated with the anode and thecathode.
 23. The electrochemical cell of claim 22 wherein thedicarbonate additive has the formula: (R¹O)C(═O)OC(═O)(OR²), wherein R¹and R² are the same or different and they can both be a hydrogen atom orone of R¹ and R² is a saturated or unsaturated organic group if theother of R¹ and R² is H or an unsaturated organic group and wherein whenany one of R¹ and R² is an unsaturated organic group, the unsaturatedorganic group contains 2 to 13 carbon atoms and has the structure(R³)(R⁴)(R⁵)C— with at least R³ being an aromatic substituent or anunsaturated organic or inorganic group and R⁴ and R⁵ being a hydrogenatom or a saturated or unsaturated hydrocarbon or heteroatom group,wherein when R¹ and R² are the same, they cannot both be saturatedorganic groups.
 24. The electrochemical cell of claim 22 wherein thedicarbonate additive is selected from the group consisting of dibenzyldicarbonate, diallyl dicarbonate, methyl benzyl dicarbonate, ethylbenzyl dicarbonate, propyl benzyl dicarbonate, butyl benzyl dicarbonate,methyl allyl dicarbonate, ethyl allyl dicarbonate, propyl allyldicarbonate, mono-allyl dicarbonate, mono-methyl dicarbonate, mono-ethyldicarbonate, mono-butyl dicarbonate, mono-propyl dicarbonate,mono-benzyl dicarbonate, cyanomethyl methyl dicarbonate, nitromethylmethyl dicarbonate, and mixtures thereof.
 25. The electrochemical cellof claim 22 wherein the nonaqueous electrolyte comprises a first solventselected from the group consisting diisopropylether,1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy, 2-methoxyethane,dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylmethylcarbonate, methylpropyl carbonate, ethylpropyl carbonate, methylacetate, tetrahydrofuran, diglyme, triglyme, tetraglyme, and mixturesthereof.
 26. The electrochemical cell of claim 22 wherein the nonaqueouselectrolyte comprises a second solvent selected from the groupconsisting of propylene carbonate, ethylene carbonate, butylenecarbonate, γ-valerolactone, γ-butyrolactone, N-methylpyrrolidinone,dimethyl sulfoxide, acetonitrile, dimethyl formamide, dimethylacetamide, and mixtures thereof.
 27. The electrochemical cell of claim22 wherein the electrolyte is selected from the group consisting ofLiPF₆, LiAsF₆, LiSbF₆, LiBF₄, LiClO₄, LiAlCl₄, LiGaCl₄, LiC(SO₂CF₃)₃,LiN(SO₂CF₃)₂, LiSCN, LiO₃SCF₂CF₃, LiC₆F₅SO₃, LiO₂CCF₃, LiSO₃F, LiNO₃,LiB(C₆H₅)₄, LiCF₃SO₃, and mixtures thereof.
 28. The electrochemical cellof claim 22 wherein the silver vanadium oxide is of the general formulaAg_(x)V₂O_(y), and wherein the silver vanadium oxide is selected fromthe group consisting of β-phase silver vanadium oxide that has in thegeneral formula x=0.35 and y=5.18, γ-phase silver vanadium oxide thathas in the general formula x=0.80 and y=5.4, ε-phase silver vanadiumoxide that has in the general formula x=1.0 and y=5.5, and mixturesthereof.
 29. A method for providing an electrochemical cell activatedwith a nonaqueous electrolyte, comprising the steps of: a) providing anegative electrode of an anode material contacted to an anode currentcollector; b) providing a positive electrode comprising a cathode activematerial contacted to a cathode current collector, wherein at least oneof the anode material and the cathode active material is mixed with adicarbonate additive prior to being contacted to the respective anodecurrent collector and cathode current collector; and c) activating theelectrochemical cell with the nonaqueous electrolyte operativelyassociated with the negative electrode and the positive electrode. 30.The method of claim 29 including providing the dicarbonate additivehaving the formula: (R¹O)C(═O)OC(═O)(OR²), wherein R¹ and R² are thesame or different and they can both be a hydrogen atom or one of R¹ andR² is a saturated or unsaturated organic group if the other of R¹ and R²is H or an unsaturated organic group and wherein when any one of R¹ andR² is an unsaturated organic group, the unsaturated organic groupcontains 2 to 13 carbon atoms and has the structure (R³)(R⁴)(R⁵)C— withat least R³ being an aromatic substituent or an unsaturated organic orinorganic group and R¹ and R⁵ being a hydrogen atom or a saturated orunsaturated hydrocarbon or heteroatom group, wherein when R¹ and R² arethe same, they cannot both be saturated organic groups.
 31. The methodof claim 29 including selecting the dicarbonate additive from the groupconsisting of dibenzyl dicarbonate, diallyl dicarbonate, methyl benzyldicarbonate, ethyl benzyl dicarbonate, propyl benzyl dicarbonate, butylbenzyl dicarbonate, methyl allyl dicarbonate, ethyl allyl dicarbonate,propyl allyl dicarbonate, mono-allyl dicarbonate, mono-methyldicarbonate, mono-ethyl dicarbonate, mono-butyl dicarbonate, mono-propyldicarbonate, mono-benzyl dicarbonate, cyanomethyl methyl dicarbonate,nitromethyl methyl dicarbonate, and mixtures thereof.
 32. The method ofclaim 29 wherein the dicarbonate additive is present in at least one ofthe negative electrode and the positive electrode in a range of about0.05% to about 5.0%, by weight.
 33. The method of claim 29 includingproviding the nonaqueous electrolyte of a first solvent selected fromthe group consisting of an ester, an ether, dialkyl carbonate, andmixtures thereof, and a second solvent selected from the groupconsisting of a cyclic carbonate, a cyclic ester, a cyclic amide, andmixtures thereof.
 34. The method of claim 29 including providing thecell as either a primary or a secondary cell.
 35. The method of claim 29wherein the electrochemical cell is a primary cell and selecting thecathode active material from the group consisting of silver vanadiumoxide, copper silver vanadium oxide, manganese dioxide, cobalt oxide,nickel oxide, fluorinated carbon, copper oxide, copper sulfide, ironsulfide, iron disulfide, titanium disulfide, copper vanadium oxide, andmixtures thereof.
 36. The method of claim 29 wherein the electrochemicalcell is a primary cell and providing the negative electrode of an anodeactive material comprised of lithium or a lithium-aluminum alloy. 37.The method of claim 29 including providing the positive electrodecomprising from about 80 to about 99 weight percent of the cathodeactive material.
 38. The method of claim 37 including providing thepositive electrode further comprising a binder material and a conductiveadditive.
 39. The method of claim 38 wherein the binder material is afluoro-resin powder.
 40. The method of claim 38 including selecting theconductive additive from the group consisting of carbon, graphitepowder, acetylene black, titanium powder, aluminum powder, nickelpowder, stainless steel powder, and mixtures thereof.
 41. The method ofclaim 29 including discharging the cell to deliver at least one currentpulse of an electrical current of a greater amplitude than that of aprepulse current immediately prior to the current pulse.
 42. The methodof claim 29 including discharging the cell to deliver at least onecurrent pulse of about 15.0 mA/cm² to about 35.0 mA/cm².
 43. The methodof claim 29 including providing the electrochemical cell as a secondarycell and selecting the cathode active material from the group consistingof oxides, sulfides, selenides, and tellurides of metals selected fromthe group consisting of vanadium, titanium, chromium, copper,molybdenum, niobium, iron, nickel, cobalt, manganese, and mixturesthereof.
 44. The method of claim 29 including providing theelectrochemical cell as a secondary cell and selecting the anodematerial from the group consisting of coke, graphite, acetylene black,carbon black, glassy carbon, hairy carbon, and mixtures thereof.
 45. Themethod of claim 29 including powering an implantable medical device withthe electrochemical cell.
 46. A method for providing an electrochemicalcell, comprising the steps of: a) providing a negative electrode oflithium; b) mixing silver vanadium oxide with a dicarbonate additive toprovide a positive electrode active mixture; c) contacting the positiveelectrode active mixture to a current collector to provide a positiveelectrode; and d) activating the negative electrode and the positiveelectrode with a nonaqueous electrolyte.
 47. The method of claim 46wherein the dicarbonate additive has the formula: (R¹O)C(═O)OC(═O)(OR²),wherein R¹ and R² are the same or different and they can both be ahydrogen atom or one of R¹ and R² is a saturated or unsaturated organicgroup if the other of R¹ and R² is H or an unsaturated organic group andwherein when any one of R¹ and R² is an unsaturated organic group, theunsaturated organic group contains 2 to 13 carbon atoms and has thestructure (R³)(R⁴)(R⁵)C— with at least R³ being an aromatic substituentor an unsaturated organic or inorganic group and R⁴ and R⁵ being ahydrogen atom or a saturated or unsaturated hydrocarbon or heteroatomgroup, wherein when R¹ and R² are the same, they cannot both besaturated organic groups.
 48. The method of claim 46 including selectingthe dicarbonate additive from the group consisting of dibenzyldicarbonate, diallyl dicarbonate, methyl benzyl dicarbonate, ethylbenzyl dicarbonate, propyl benzyl dicarbonate, butyl benzyl dicarbonate,methyl allyl dicarbonate, ethyl allyl dicarbonate, propyl allyldicarbonate, mono-allyl dicarbonate, mono-methyl dicarbonate, mono-ethyldicarbonate, mono-butyl dicarbonate, mono-propyl dicarbonate,mono-benzyl dicarbonate, cyanomethyl methyl dicarbonate, nitromethylmethyl dicarbonate, and mixtures thereof.
 49. The method of claim 46wherein the dicarbonate additive is present in the positive electrodeactive mixture in a range of about 0.05% to about 5.0%, by weight.